The behaviour of a pool of flagellar precursors, assayed by the ability of cells to regenerate flagella in the absence of de novo protein synthesis, has been examined during organelle morphogenesis in the biflagellate alga Chlamydomonas. The results demonstrate that flagellar elongation can continue even when this pool is apparently empty and suggest that 2 sources of precursors are available to the regenerating flagella: those pre-existing in the cellular pool and those synthesized de novo. Further evidence for this was obtained by subjecting regenerating cells to pulses of cycloheximide. Cells exposed to this drug during the first 60 min post deflagellation formed only half-length (5-mum) flagella, whereas a pulse administered after this point allowed the formation of longer flagella and suggested that some de novo protein synthesis was required for the formation of full-length flagella, although it was not a prerequisite for the initiation of regeneration. In addition, it was found that, subsequent to the removal of the cycloheximide, flagellar regeneration did not recommence immediately, but was delayed for a period of approximately 45 min, irrespective of length of flagella formed prior to drug inhibition. The nature of this cycloheximide-induced delay is unclear and certain alternatives, based on the exhaustion of structural/regulatory components are considered. Although it is not possible to distinguish between these alternatives, tubulin is not the limiting component, since a pool of this protein is present when flagellar elongation is prevented by cycloheximide.